Traffic Loading 'Fingerprinting' of Bridges of Assessment Purposes

The paper describes a field trial of the use of a simple statistical ‘extrapolation' technique for the determination of design load effects in existing bridges. Deflections were measured directly using lasers in the Foyle Bridge, and data were recorded for 155 daily 48-min samples. As only traffic load effects were of interest, wind-induced defections were removed by Fast Fourier transform analysis and temperature-induced deflections were removed through identification of traffic-free periods. A simple linear regression analysis using probability paper has been employed to determine the parameters which characterise the statistical distribution. The cumulative distribution function was then used to estimate the level of deflection with a 1000-year return period. Empirically derived formulas have been utilised to determine the variability in the 1000-year estimates and to calculate a design deflection which allows for this.